1. Field of the Invention
Aspects of the present invention relate to an electrolyte for a lithium ion secondary battery and a lithium ion secondary battery comprising the electrolyte. More specifically, aspects of the present invention relates to an electrolyte for a lithium ion secondary battery that uses at least one aromatic phosphate compound to achieve improved characteristics in terms of overcharge safety, high-temperature safety and excellent life cycle characteristics, as well as a lithium ion secondary battery comprising the electrolyte.
2. Description of the Related Art
During the initial charge of a lithium ion battery, a solid electrolyte interface (SEI) film, also called a passivation layer, is formed on the surface of a graphite negative electrode. Once the SEI film is formed, lithium ions are prevented from undergoing side reactions with the graphite negative electrode or other materials and the amount of the lithium ions in the electrolyte is reversibly maintained, resulting in the maintenance of stable charge/discharge characteristics. The SEI film slowly collapses during high-temperature exposure in a fully charged state with the passage of time, and as a result, the surface of the negative electrode is exposed. The exposed portions of surface of the negative electrode continuously react with the surrounding electrolyte to release gases. This continuous gas release increases the internal pressure of the battery. Particularly, the thickness of a prismatic battery increases because of the gas release.
Further, when a lithium ion secondary battery is overcharged, excessive precipitation and insertion of lithium occur in both the positive and negative electrodes of the battery, resulting in thermal instability of the electrodes. This thermal instability may induce rapid exothermic thermal decomposition reactions between the electrodes and the electrolyte. In an extreme case, thermal runaway may occur, posing a danger of rupture of the battery as well as fire.
To solve the above problems, methods have been developed for the preparation of electrolytes using aromatic compounds as redox shuttle additives. For example, U.S. Pat. No. 5,709,968 discloses a non-aqueous lithium ion battery using a benzene-related compound such as 2,4-difluoroanisole to interrupt the overcharge current and therefore prevent thermal runaway occurring because of the overcharge current. Further, U.S. Pat. No. 5,879,834 describes a method for improving the safety of a battery by incorporating small amounts of suitable aromatic additives such as biphenyl, 3-chlorothiophene and furan into an electrolyte. The additives are electrochemically polymerized at abnormally high voltages to increase the internal resistance of the battery. These redox shuttle additives increase the internal temperatures of batteries at the early stages due to heat generated by exothermic oxidation reactions. The early increased temperature serves to rapidly and uniformly block pores of separators inserted in the batteries to suppress overcharge reactions. Another function of the redox shuttle additives is that polymerization of the additives on the surface of positive electrodes consumes the overcharge current during overcharge to protect the batteries. However, the overcharge current cannot be sufficiently removed by the polymerization of the additives and large amounts of gases are still released from the decomposition of the additives due to redox reactions and therefore continue to cause severe swelling of the batteries.